The present invention relates to a wax-pellet thermostat for an engine cooling system for an automobile.
FIG. 4 shows a conventional cooling system in an automotive engine having a thermostat 1. The thermostat 1 comprises a thermo actuator 38, a primary valve 5 and a secondary valve 17. The thermostat 1 is disposed in a coolant discharge passage between an upper outlet 26 of water jackets 24 of an engine and an upper inlet 31 of a radiator 30. Namely, an upper outlet 26 of the water jackets 24 is communicated with the upper inlet 31 of the radiator 30 through a thermostat housing 27, a cap 28 and a passage 29. An outlet of a water pump 25 is connected to a lower inlet of the water jackets 24. A lower outlet 32 of the radiator 30 is connected to an inlet 33 of the water pump 25 through a passage 34. A bypass passage 36 is provided between a valve opening of the secondary valve 17 and a junction 35 of the passage 34.
During engine warmup at the initial stage, since the engine is cold, the primary valve 5 of the thermostat 1 in the thermostat housing 27 is closed to cut off the flow of coolant through the radiator 30. Coolant is circulated by the water pump 25 passing through the water jackets 24, thermostat housing 27, and bypass passage 36, as indicated by arrows, so that the temperature of the coolant rapidly rises.
When the engine reaches its normal operating temperature, and the temperature of the coolant reaches a predetermined valve-opening temperature of the thermostat 1, the primary valve 5 of the thermostat opens. Thus, a part of coolant flows from the thermostat housing 27 into the radiator 30 through the passage 29 and flows from top to bottom in the radiator 30, and drawn into the water jackets 24 through the passage 34 by the water pump 25 together with the coolant from the bypass passage 36.
In practice, however, the heat sensitivity of the thermo-actuator 38 of thermostat 1 is low, so that the temperature rising rate of the thermostat is slow compared with the temperature rising rate of the coolant. Under certain conditions, the thermostat 1 does not open at the predetermined valve-opening temperature. For example, the thermostat 1 opens When the temperature of the coolant reaches a higher temperature by 15.degree. C., or 25.degree. C. in an extreme case, than the predetermined temperature. In such a case, since the thermostat 1 suddenly opens and the low temperature coolant flows from the radiator 30 into the water jackets 24 through the water pump 25, the temperature of coolant in the water jackets 24 is rapidly lowered by the coolant from the the radiator 30 which causes the thermostat to close again.
If the peak of the heat overshoot occurring in the initial opening period of thermostat and the surge of pressure occurring at the closing of the thermostat simultaneously occur and repeat, cracks of the water jackets and cylinder head, breakdowns of the thermostat and radiator, and other problems will occur. This will cause serious engine troubles.
In order to reduce the heat overshoot and the surge pressure at the initial state of the engine warmup, another engine cooling system shown in FIG. 5 is mainly employed in automobiles at present. In the cooling system, the thermostat 1 is provided in the inlet side passage of the water jackets. In the closing state of the thermostat 1 at an initial stage, the coolant circulates passing through the water jacket 24, the bypass passage 36.
In the closed state of the primary valve 5 of the thermostat 1, the pressure of the coolant from the radiator 30 is applied to the primary valve 5 of the thermostat 1 to urge it to open. In the conventional cooling system, the coolant leaks about 0.2 liter per minute under the pressure of the coolant of 0.8 kg/cm.sup.2, passing through the primary valve 5. Consequently, the thermostat 1 is cooled by the low temperature coolant, so that the thermostat does not open at the predetermined temperature. As a result, the engine is not properly cooled, thereby reducing the thermal efficiency of the engine and occurring the heat overshoot and the surge pressure.